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Abstract:

A method controls a direct-injection gaseous-fuelled internal combustion
engine system to improve the conversion efficiency of an SCR converter
that is operative to reduce levels of NOx. The method comprises detecting
when the internal combustion engine is idling and timing the injection of
a first quantity of fuel to begin injection when the engine's piston is
near top dead center; and controlling the temperature of exhaust gas to
be above a predetermined temperature that is defined by an operating
temperature range that achieves a desired conversion efficiency for the
selective catalytic reduction converter, by: (a) timing injection of the
gaseous fuel to begin after timing for injection the first quantity of
fuel, and (b) increasing exhaust gas temperature by increasing a delay in
timing for injecting the gaseous fuel, while limiting the delay to keep
concentration of unburned fuel exiting the combustion chamber below a
predetermined concentration.

Claims:

1. A method of controlling an internal combustion engine comprising a
combustion chamber defined by a cylinder and a piston reciprocable within
said cylinder, said piston being connected to a crankshaft that rotates
when said piston reciprocates, and an injector for injecting a gaseous
fuel directly into said combustion chamber, wherein an exhaust gas
exiting from said combustion chamber is received in a selective catalyst
reduction (SCR) converter that is operative to reduce levels of NOx in
said exhaust gas by converting NOx into nitrogen and water, said method
comprising the steps of: (A) detecting at least one engine parameter
indicative of when said internal combustion engine is idling; and (B) in
an engine cycle, when determining that said internal combustion engine is
idling: (i) timing an injection of a first quantity of fuel to begin when
said piston is near top dead center; and (ii) controlling temperature of
exhaust gas exiting said combustion chamber to be above a predetermined
temperature that is defined by an operating temperature range that
achieves a desired conversion efficiency for said selective catalytic
reduction converter, by: (a) timing beginning of an injection of said
gaseous fuel to be after timing for injecting said first quantity of
fuel, and (b) increasing exhaust gas temperature by increasing a delay in
timing for injecting said gaseous fuel, while limiting said delay to keep
concentration of unburned fuel exiting said combustion chamber below a
predetermined concentration.

2. The method of claim 1 further comprising injecting said first quantity
of fuel in a plurality of pulses introduced sequentially into said
combustion chamber.

3. The method of claim 2 wherein each one of said pulses has the same
duration.

4. The method of claim 1 wherein one parameter indicative of when said
engine is idling is said engine's speed.

5. The method of claim 1 wherein one parameter indicative of when said
engine is idling is a total fuelling amount.

6. The method of claim 1 further comprising, when determining from said
at least one engine parameter that said engine has transitioned from
idling to load, gradually advancing both timing for beginning injection
of said first quantity of fuel and timing for beginning injection of said
gaseous fuel until each begins before said piston is at top dead center
with each timing predetermined based on engine speed and respective
commanded quantities of fuel based on total fuel energy required by
engine load.

7. The method of claim 2 further comprising, when determining from said
at least one engine parameter that said engine has transitioned from
idling to load, gradually advancing both timing for beginning injection
of each one of said plurality of pulses and timing for beginning
injection of said gaseous fuel, and decreasing elapsed time between said
plurality of pulses, until said plurality of pulses has merged into a
single pulse and timing for beginning injection of said first quantity of
fuel and said gaseous fuel is before said piston is at top dead center
with each timing predetermined based on engine speed and respective
commanded quantities of fuel based on total fuel energy required by
engine load.

8. The method of claim 1 wherein said delay in timing, measured in
degrees of crank angle rotation is between 14 and 25 degrees after top
dead center.

9. The method of claim 1 wherein said predetermined concentration of
unburned fuel exiting said combustion chamber is less than or equal to
1000 ppm.

10. The method of claim 1 wherein said predetermined temperature of said
exhaust gas exiting said combustion chamber is equal to or higher than
200 degrees Celsius.

11. The method of claim 1 further comprising ending injection of said
first quantity of fuel at a timing when said crankshaft angle of rotation
is within 1 degree before or after beginning said injection of said
gaseous fuel.

12. The method of claim 1 further comprising beginning said injection of
said first quantity of fuel when said crankshaft is positioned between 2
crank angle degrees before top dead center and 5 crank angle degrees
after top dead center.

13. The method of claim 1 wherein said gaseous fuel is selected from the
group consisting of natural gas, methane, propane, butane, hydrogen, and
mixtures thereof.

14. The method of claim 1 wherein said first quantity of fuel is a fuel
that is the same as said gaseous fuel and said internal combustion engine
further comprises an igniter disposed within said combustion chamber for
igniting said fuel.

15. The method of claim 1 wherein said first quantity of fuel is a fuel
that auto-ignites in said combustion chamber.

16. The method of claim 15 wherein said first quantity of fuel is
selected from the group consisting of diesel fuel, dimethylether,
bio-diesel, and kerosene.

17. A method of controlling an internal combustion engine comprising a
combustion chamber defined by a cylinder and a piston reciprocable within
said cylinder, said piston being connected to a crankshaft that rotates
when said piston reciprocates, and an injector for injecting a gaseous
fuel directly into the combustion chamber, wherein an exhaust gas exiting
from said combustion chamber is received in a selective catalyst
reduction (SCR) converter that is operative to reduce levels of NOx in
said exhaust gas by converting NOx into nitrogen and water, said method
comprising the steps of: (A) detecting said engine's speed and total
fuelling amount; (B) when determining from said engine speed and total
fuelling amount that said engine is idling: (i) timing an injection of a
first quantity of fuel injected directly into said combustion chamber in
two pulses, to begin injection of a first pulse when said piston is near
top dead center and to begin injection of a second pulse after ending
said injection of said first pulse, (ii) controlling temperature of
exhaust gas exiting said combustion chamber to be above a predetermined
temperature that is defined by an operating temperature range that
achieves a desired conversion efficiency for said selective catalytic
reduction converter by: (a) timing beginning of an injection of said
gaseous fuel directly into the combustion chamber to be after timing for
injection of said two pulses, and (b) increasing exhaust gas temperature
by increasing a delay in timing for injecting said gaseous fuel, while
limiting said delay to keep concentration of unburned fuel exiting said
combustion chamber below a predetermined concentration, wherein timing
for beginning injection of said second pulse is adjusted to be near
timing for beginning injection of said gaseous fuel.

18. The method of claim 17 wherein said delay in timing, measured in
degrees of crank angle rotation is between 14 and 25 degrees after top
dead center.

19. The method of claim 17 wherein said predetermined concentration of
unburned fuel exiting said combustion chamber is less than or equal to
1000 ppm.

20. The method of claim 17 wherein said predetermined temperature of said
exhaust gas exiting said combustion chamber is equal to or higher than
200 degrees Celsius.

21. The method of claim 17 further comprising ending injection of said
first quantity of fuel at a timing when said crankshaft angle of rotation
is within 1 degree before or after beginning said injection of said
gaseous fuel.

22. The method of claim 17 further comprising beginning said injection of
said first quantity of fuel when said crankshaft is positioned between 2
crank angle degrees before top dead center and 5 crank angle degrees
after top dead center.

23. The method of claim 17 wherein each one of said pulses has the same
duration.

24. The method of claim 17 wherein said gaseous fuel is selected from the
group consisting of natural gas, methane, propane, butane, hydrogen, and
mixtures thereof.

25. The method of claim 17 wherein said first quantity of fuel is a fuel
that is the same as said gaseous fuel and said internal combustion engine
further comprises an igniter disposed within said combustion chamber for
igniting said fuel.

26. The method of claim 17 wherein said first quantity of fuel is a fuel
that auto-ignites in said combustion chamber.

27. The method of claim 26 wherein said first quantity of fuel is
selected from the group consisting of diesel fuel, dimethylether,
bio-diesel, and kerosene.

28. The method of claim 17 further comprising, when determining from said
engine speed and total fuelling amount that said engine has transitioned
from idling to load, gradually advancing both timing for beginning
injection of each one of two pulses and timing for beginning injection of
said gaseous fuel, decreasing elapsed time between the end of said first
pulse and the beginning of said gaseous fuel injection and decreasing the
amount of fuel injected in said second pulse, until a single pulse is
injected into the combustion chamber and timing for beginning injection
of said single pulse and said gaseous fuel is before said piston is at
top dead center with each timing predetermined on engine speed and
respective commanded quantities of fuel based on total fuel energy
required by engine load.

Description:

CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This application is a continuation of International Application No.
PCT/CA2011/000442, having an international filing date of Apr. 15, 2011,
entitled "Method Of Controlling A Direct-Injection Gaseous-Fuelled
Internal Combustion Engine System With A Selective Catalytic Reduction
Converter". The '442 international application claimed priority benefits,
in turn, from Canadian Patent Application No. 2,698,342 filed on Apr. 20,
2010, and from Canadian Patent Application No. 2,702,246 filed on May 7,
2010, each of which is hereby incorporated by reference herein in its
entirety.

FIELD OF THE INVENTION

[0002] The present invention relates to a method of controlling a
direct-injection gaseous-fuelled internal combustion engine system with a
selective catalytic reduction converter to reduce emissions of NOx when
the engine is idling.

BACKGROUND OF THE INVENTION

[0003] Presently, most over-the-road heavy vehicles are fuelled by
gasoline or diesel fuel. Because both gasoline and diesel-fuelled
internal combustion engines generate a significant amount of pollutants
such as oxides of nitrogen (NOx) and particulate matter (PM), engine
manufacturers have been searching for best ways to improve their engines
to comply with the new government regulatory standards which are becoming
progressively more stringent with respect to the allowed levels of
pollutants in tailpipe emissions.

[0004] For diesel-cycle engines one approach that shows a significant
improvement in reducing the levels of pollutants in tailpipe emissions
involves substituting a part or all the diesel fuel with cleaner burning
gaseous fuels such as natural gas, pure methane, ethane, liquefied
petroleum gas, lighter flammable hydrocarbon derivatives, hydrogen, and
blends of such fuels. Gaseous fuels are generally defined herein as fuels
that are gaseous at atmospheric pressure and zero degrees Celsius.
Whereas liquid fuels such as diesel are injected at very high pressures
in order to atomize the fuel, gaseous fuels can be injected into an
engine's combustion chamber at lower pressures because no extra energy is
required for fuel atomization. An advantage of using the diesel-cycle and
substituting a gaseous fuel for diesel fuel is this approach can preserve
the high efficiency and high torque of the conventional diesel engines,
while reducing pollutant levels in tailpipe emissions.

[0005] However, some modifications are required to a conventional diesel
engine to allow gaseous fuels to be substituted for diesel fuel. In a
conventional diesel engine, the heat produced by the mechanical
compression of the fuel and air mixture auto-ignites the liquid diesel
fuel charge at or near the end of the piston's compression stroke. Other
liquid fuels such as dimethyl ether, bio-diesel, and kerosene will also
auto-ignite at the temperatures and pressures within the combustion
chamber generated by the compression of the charge within the combustion
chamber. However, under the same temperature and pressure conditions
generated by the compression of the charge within the combustion chamber,
gaseous fuels such as natural gas will not reliably auto-ignite.
Therefore, in order to reliably burn a gaseous fuel in a conventional
compression ignition engine with the same compression ratio as a diesel
engine, an igniter is required within the combustion chamber to assist
with ignition of the gaseous fuel, such as a hot surface provided by a
glow plug, a spark plug, or a fuel injection valve for introducing a fuel
that will reliably auto-ignite, acting as a pilot fuel. The pilot fuel
can be a small quantity of diesel fuel, whereby the auto-ignition of the
diesel fuel triggers the ignition of gaseous fuel.

[0006] While gaseous fuels are generally cleaner burning than conventional
liquid fuels, tailpipe emissions from gaseous-fuelled engines can be
further improved to reduce the levels of NOx by applying a treatment
called Selective Catalytic Reduction ("SCR") to the gases exhausted from
the engine. In an SCR converter, ammonia is injected into the exhaust
stream upstream of the SCR catalyst as a reduction agent. The ability of
ammonia as a reductant to achieve a significant reduction of NOx has been
proven for stationary power applications and therefore has been used in
diesel-fuelled engines. Other forms of ammonia can be used, such as urea,
aqueous, gaseous or liquid ammonia. Using an SCR converter, the SCR
catalyst facilitates the reaction between ammonia and NOx to produce
water and nitrogen gas.

[0007] However, the applicants have found that combining an SCR converter
with a gaseous-fuelled engine did not always achieve the same NOx
conversion rates. Under some conditions, especially when the engine is
idling, it was found that the temperature of the exhaust gas exiting the
combustion chamber was significantly lower than the temperatures normally
found under higher speed engine operation. To maintain a high NOx
conversion rate it was determined that the temperature of the catalytic
bed in the SCR converter is preferably above a predetermined temperature
which can vary depending upon the composition of the catalyst. Generally,
if the temperature of the exhaust gas exiting the combustion chamber is
maintained above 200 degrees Celsius, acceptable NOx conversion rates are
achieved.

[0008] For conventional diesel engines there are many known approaches for
increasing the exhaust gas temperature, but there are particular
characteristics of gaseous-fuelled engines that prevent the simple
transfer of these approaches. For example, some approaches result in
unburned fuel being introduced into the exhaust stream, and gaseous
fuels, such as natural gas, which consists mostly of light hydrocarbons
(methane in particular), do not readily oxidize in the diesel oxidation
catalyst of the after-treatment system, especially at lower temperatures,
and therefore do not generate heat to be used by the aftertreatment
system.

[0009] Therefore there are special considerations that need to be taken
into account to develop a successful engine system that uses a gaseous
fuel and a SCR converter for reducing levels of NOx in the tailpipe
emissions.

BRIEF SUMMARY OF THE INVENTION

[0010] A control method is provided for an internal combustion engine
comprising a combustion chamber defined by a cylinder and a piston
reciprocable within the cylinder, the piston being connected to a
crankshaft that rotates when the piston reciprocates, and an injector for
injecting a gaseous fuel directly into the combustion chamber. The
exhaust gas exiting from the combustion chamber is received in a
selective catalyst reduction (SCR) converter that is operative to reduce
levels of NOx in the exhaust gas by converting NOx into nitrogen and
water.

[0011] The method comprises the steps of detecting at least one engine
parameter indicative of when the internal combustion engine is idling
and, in an engine cycle, when determining that the internal combustion
engine is idling timing the injection of a first quantity of fuel to
begin injection when the piston is near top dead center and controlling
temperature of exhaust gas exiting the combustion chamber to be above a
predetermined temperature that is defined by an operating temperature
range that achieves a desired conversion efficiency for the selective
catalytic reduction converter. The temperature of the exhaust gas is
controlled by timing the beginning of injection of the gaseous fuel to be
after the injection the first quantity of fuel, and increasing exhaust
gas temperature by increasing a delay in timing for injecting the gaseous
fuel, while limiting the delay to keep the concentration of unburned fuel
exiting the combustion chamber below a predetermined concentration.

[0012] One parameter indicative of when the engine is idling can be the
engine's speed. Another parameter indicative of when the engine is idling
can be a total fuelling amount. Also, a controller could read the values
of both these parameters from a two-axis map to determine when the engine
is idling.

[0013] In some embodiments of the present method, the fuel injector
injects the first quantity of fuel in a plurality of pulses introduced
sequentially into the combustion chamber. Each one of the pulses can have
the same duration, or they can be different in duration.

[0014] If a controller determines from at least one engine parameter, for
example the engine speed or total fuelling amount, that the engine has
transitioned from idling to load, it gradually advances both timing for
beginning injection of the first quantity of fuel and timing for
beginning injection of the gaseous fuel until each begins before the
piston is at top dead center with each timing predetermined based on
engine speed and respective commanded quantities of fuel based on total
fuel energy required by engine load.

[0015] If the first quantity of fuel is injected into the combustion
chamber in a plurality of pulses and the controller determines that the
engine has transitioned from idling to load, it gradually advances both
timing for beginning injection of each one of the plurality of pulses and
timing for beginning injection of the gaseous fuel, and decreasing
elapsed time between the plurality of pulses, until the plurality of
pulses has merged into a single pulse and until the timing for beginning
injection of the first quantity of fuel and the gaseous fuel is advanced
to occur before the piston is at top dead center and the injection
timings are predetermined based on engine speed and respective commanded
quantities of fuel based on total fuel energy required by engine load.

[0016] In a preferred embodiment, when determining from the engine speed
and total fuelling amount that the engine is idling, the method comprises
the step of timing an injection of a first quantity of fuel in two pulses
to begin injection of a first pulse when the piston is near top dead
center and to begin injection of a second pulse after ending the
injection of the first pulse. The controller controls the temperature of
exhaust gas exiting the combustion chamber to be above a predetermined
temperature that is defined by an operating temperature range that
achieves a desired conversion efficiency for the selective catalytic
reduction converter by timing beginning of an injection of the gaseous
fuel directly into the combustion chamber to be after timing for
injection of the two pulses, and increasing exhaust gas temperature by
increasing a delay in timing for injecting the gaseous fuel, while
limiting the delay to keep concentration of unburned fuel exiting the
combustion chamber below a predetermined concentration. The controller
adjusts the timing for beginning injection of the second pulse to be
generally near beginning of the gaseous fuel injection. The second pulse
can have the same duration as the first pulse or they can be different in
duration.

[0017] When the first quantity of fuel is injected in two pulses and the
controller determines from the engine speed and total fuelling amount
that the engine has transitioned from idling to load, it gradually
advances both timing for beginning injection of each one of two pulses
and timing for beginning injection of the gaseous fuel, decreases elapsed
time between the end of the first pulse and the beginning of the gaseous
fuel injection and decreases the amount of fuel injected in the second
pulse, until a single pulse is injected into the combustion chamber and
until the timing for beginning injection of the single pulse and the
gaseous fuel is before the piston is at top dead center with each timing
predetermined based on engine speed and respective commanded quantities
of fuel based on total fuel energy required by engine load.

[0018] In preferred embodiments, for example for a 15 liter
direct-injection natural gas internal combustion engine ignited by a
diesel fuel, the delay in timing for injecting the gaseous fuel, measured
in degrees of crank angle rotation can be between 14 and 25 degrees after
top dead center. In such embodiments the controller can end injection of
the first quantity of fuel at a timing when the crankshaft angle of
rotation is within 1 degree before or after of beginning the injection of
the natural gas so that the ignition of the diesel fuel can warm up the
combustion chamber and thereby transfer the heat to the natural gas
injected into the combustion chamber after the diesel. The beginning of
the injection of the first quantity of fuel can start when the crankshaft
is positioned between 2 crank angle degrees before top dead center and 5
crank angle degrees after top dead center.

[0019] For many engines, the predetermined concentration of unburned fuel
exiting the combustion chamber that is acceptable for an efficient
operation of the selective catalytic reduction converter is 1000 ppm. In
preferred embodiments the concentration of unburned fuel in the exhaust
can be below between 200 and 300 ppm.

[0020] Since selective catalytic reduction converters require a
temperature of at least 200 degrees Celsius to operate efficiently, the
predetermined temperature of the exhaust gas exiting the combustion
chamber according to the present method is generally equal to or higher
than 200 degrees Celsius.

[0021] The gaseous fuel injected directly into the combustion chamber is
selected from the group consisting of natural gas, methane, propane,
butane, hydrogen, and mixtures thereof.

[0022] When the first quantity of fuel injected into the combustion
chamber is a fuel that is the same as the gaseous fuel, the internal
combustion engine comprises an igniter disposed within the combustion
chamber for igniting the fuel, for example a glow plug, a spark plug or a
hot surface.

[0023] In other embodiments, the first quantity of fuel injected into the
combustion chamber is a fuel that auto-ignites in the combustion chamber,
for example a fuel selected from the group consisting of diesel fuel,
dimethylether, bio-diesel, and kerosene.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a schematic view of a direct-injection gaseous-fuelled
internal combustion engine system comprising an exhaust after-treatment
subsystem and an exhaust gas recirculation loop.

[0025]FIG. 2 is a schematic representation of the fuel injection timings
for a gaseous-fuelled internal combustion engine when the engine is
operating at idle, mid speed or higher speeds according to a conventional
method of fuel injection control known in the prior art.

[0026]FIG. 3 is a schematic representation of the fuel injection timings
when the engine operates at idle according to the present method whereby
the exhaust gas temperature is increased and maintained at the required
temperature for improving NOx conversion rates in the SCR converter.

[0027]FIG. 4 is an illustration of the fuel injection timing zones for an
engine test cycle when the present fuel injection control strategy was
applied and it also illustrates the SCR catalyst bed temperature recorded
during testing.

[0028]FIG. 5 is an illustration of the engine's speed-fuelling map that
shows the range of engine speed and total fuelling amount where the
present fuel injection strategy can be applied.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

[0029]FIG. 1 shows a schematic view of a direct-injection gaseous-fuelled
internal combustion engine system comprising an exhaust after-treatment
subsystem and an exhaust gas recirculation loop. Herein
"direct-injection" is used to refer to the injection of fuel directly
into the combustion chamber of an internal combustion engine, which is an
approach that is technically distinct from engines that inject fuel into
an engine's intake manifold or into the intake ports on the manifold side
of the engine's intake valves. With direct-injection engines the fuel can
be injected later in the engine cycle, thereby avoiding fuelling and
compression ratio limitations associated with avoiding engine knock
("pre-mature detonation of the fuel"). Conversely, this generally allows
direct-injection engines to employ higher compression ratios, and achieve
higher efficiencies and power outputs compared to other engines with the
same displacement. The disclosed method can be used with engines that
inject gaseous fuel directly into the combustion chamber through an
injector. The gaseous fuel can be ignited by an ignition means which can
be a spark plug, a glow plug, a hot surface or a pilot fuel that
auto-ignites inside the combustion chamber. When gaseous fuel ignition is
assisted by a pilot fuel, the pilot fuel is preferably introduced
directly into the combustion chamber by a separate injector. In some
embodiments the gaseous fuel injector and the pilot fuel injector are
integrated into a single assembly, but with separate passages for the
gaseous and pilot fuels so that the two injectors are independently
operable to separately inject each fuel at different times. The schematic
view shown in FIG. 1 is not to scale, with some parts shown larger
relative to the other parts to better illustrate their function. The
disclosed direct-injection internal combustion engine has at least one
cylinder, a piston being reciprocable within the cylinder in known
fashion, and having a crankshaft connected to the piston which is
rotatable by the reciprocal movement of the piston within the cylinder.
In this disclosure, the fuel injection into the combustion chamber is
described with reference to crank angle degrees before or after top dead
center (TDC) which represent the position of the crankshaft relative its
position when the piston is at TDC. The piston is at TDC when it has
reached the end of a compression stroke and is about to begin an
expansion stroke, more specifically when the piston is closest to the
cylinder head.

[0030] Referring to FIG. 1, internal combustion engine system 100 shows an
illustrative embodiment of a direct-injection gaseous-fuelled engine that
uses a pilot fuel, for example diesel fuel, to assist in igniting the
gaseous fuel injected into a combustion chamber. Internal combustion
engine system 100 generally comprises engine 110, diesel fuel delivery
subsystem 112, gaseous fuel delivery subsystem 114, and controller 116.
The engine system further comprises air intake line 118 and exhaust gas
line 120. Some of the exhaust gas exiting the engine in direction 122 is
directed through exhaust gas recirculation loop 124 in direction 126 and
through valve 128 into air intake line 118 where it is mixed with intake
air flowing through air intake line 118. The mix of fresh intake air and
recirculated exhaust gas is delivered to the intake ports of engine 110
in the direction shown by arrow 130. The exhaust gas exiting engine 110,
which is not recirculated, flows in direction shown by arrow 132 through
turbocharger 138 and on to line 134 which is connected to exhaust gas
after-treatment subsystem 140 and from there the exhaust gas is released
into the atmosphere through the tailpipe. Turbocharger 138 preferably has
a variable geometry, as known to persons familiar with the technology
involved here, but the disclosed method can also be applied to engines
without a turbocharger.

[0032] Controller 116 can be integrated into a vehicle controller or it
can be a separate controller that communicates with the vehicle
controller. Controller 116 controls diesel fuel injection delivery
subsystem 112, gaseous fuel delivery subsystem 114 and exhaust gas
after-treatment subsystem 140 based on the detected engine operation
state. Controller 116 receives information about at least one engine
parameter indicative of the engine operation state, such as the engine
speed and total fuelling amount. Based on the engine maps stored in its
memory, controller 116 can determine when the engine is idling as further
explained below in relation to FIGS. 4 and 5.

[0033]FIG. 2 illustrates a schematic representation of the fuel injection
timings for a direct-injection gaseous-fuelled internal combustion engine
similar to the one illustrated in FIG. 1, when the engine is operating at
idle (1), mid-speed (2) or higher speeds (3). The approach to timing the
start of injection is adapted from the known approach used for injecting
diesel into diesel-fuelled engines, except that instead of injecting just
diesel fuel, an initial pulse of diesel pilot fuel is injected followed
by a larger pulse of gaseous fuel. Herein "idle" is used to refer to the
state of an engine operating at low speeds (typically around 700 rpm for
compression ignition engines, but idle speed can vary depending upon
engine design) when the only load served by the engine is generated by
friction and parasitic loads. When the engine operates at idle (1)
controller 116 controls fuel delivery subsystems 112 and 114 to inject a
quantity of diesel fuel 210 directly into the combustion chamber of the
engine before top dead center and to inject a quantity of gaseous fuel
220 directly into the combustion chamber shortly after the diesel fuel
injection and near top dead center. The timing for injecting the fuel and
the fuel quantity is optimized to maintain a predetermined engine idling
speed when no productive loads are served by the engine and the only load
is generated by friction and parasitic loads. During the engine's
mid-speed operation (2), when the engine speed increases compared to when
the engine operates at idle, the timing of diesel fuel injection 230 and
of gaseous fuel injection 240 occur earlier in the engine cycle relative
to the top dead center when compared to the timing of fuel injection at
idle. When the engine speed increases further to higher speeds, as shown
in example (3) of FIG. 2, the timing of diesel fuel injection 250 and of
gaseous fuel injection 260 occurs even earlier in the engine cycle
relative to the top dead center.

[0034] In the prior art method described above, like with diesel-fuelled
engines, in all operation modes diesel fuel is injected before top dead
center and the start of the gaseous fuel injection generally occurs near
or before top dead center. As engine speed increases, the start of diesel
fuel injection and of gaseous fuel injection occurs earlier in the engine
cycle. For example, for a 15 liter direct-injection gaseous-fuelled
engine using diesel pilot ignition, the earliest timing of diesel fuel
injection can be between 10 to 20 degrees crank angle before TDC when the
engine operates at high speeds. In the prior art method illustrated in
FIG. 2, diesel fuel is injected into the combustion chamber near the
start of the gaseous fuel injection so that when the diesel pilot fuel
ignites, it generates enough heat for heating the combustion chamber and
effectively igniting the gaseous fuel that is introduced into the
combustion chamber sequentially after the pilot fuel.

[0035] When an igniter is employed, such as, for example, a glow plug or
other hot surface, or a spark plug, a pilot fuel is not needed.
Nevertheless, in some embodiments the same fuelling strategy can be used
if the igniter is employed to ignite a pilot quantity of gaseous fuel
which in turn ignites the main quantity of gaseous fuel injected in
respective pulses 220, 240 and 260.

[0036] The presently disclosed improved method of controlling fuel
injection when the engine operates at idle is illustrated in FIG. 3 which
shows the fuel injection timings for four different embodiments A to D,
all relating to fuel injection strategies for when a gaseous-fuelled
direct-injection internal combustion engine is idling. Based on the
embodiments illustrated in FIG. 3 and described in more detail below, the
present method can be implemented on a gaseous-fuelled internal
combustion engine that employs a pilot fuel such as diesel for assisting
ignition or engines that employ other means of assisting ignition, such
as, for example a glow plug or another hot surface, or a spark plug. In
such cases gaseous fuel pulses can replace diesel pulses in the
injections described below while still following the injection patterns
illustrated in embodiments A through D of FIG. 3.

[0037] In the embodiments of the presently disclosed method, gaseous fuel
injection starts later in the engine cycle compared to the gaseous fuel
injection timing practiced in the prior art conventional methods that
followed conventional approaches employed by diesel-fuelled engines, as
illustrated in FIG. 2. By delaying the injection of the gaseous fuel,
more heat is transferred to the exhaust gas exiting the combustion
chamber and this heat is carried into the SCR catalyst bed, helping to
maintain it at a higher temperature when the engine is idling. In
embodiments in which a pilot fuel is employed as the means for assisting
ignition, because of the relatively low energy requirements for
sustaining the engine speed when idling, most of the energy needed to
sustain idling is provided by the combustion of the pilot fuel, and much
of the energy from burning the gaseous fuel is converted into heat. In
embodiment A of the presently disclosed method the timing for injecting
first quantity of fuel 310 directly into the combustion chamber is set to
begin when the piston is near top dead center, and the combustion of this
fuel contributes mostly to overcoming friction and parasitic loads to
sustain engine idling speed. The injection of first quantity of fuel 310
can end near the timing for beginning gaseous fuel injection 312. Gaseous
fuel is injected directly into the combustion chamber and gaseous fuel
injection 312 begins sequentially after the timing of the injection of
the first quantity of fuel 310 and after top dead center. With this
approach, the timing for the combustion of the gaseous fuel is adjusted
to increase the exhaust gas temperature to at least 200 degrees Celsius,
which maintains the temperature inside SCR converter 142 at an operating
range that improves NOx conversion efficiency. That is, the temperature
of the exhaust gas exiting the combustion chamber is increased by
increasing a delay in timing for beginning injection of gaseous fuel
injection 312, but unlike with conventional diesel engines, for
gaseous-fuelled engines the length of this delay is limited to keep
concentration of unburned fuel exiting the combustion chamber below a
predetermined concentration, generally below 1000 ppm and, in preferred
embodiments, below a concentration of between 200 and 300 ppm.

[0038] In preferred embodiments, injection of first quantity of fuel 310
ends within 1 degree crank angle before or after of timing for beginning
gaseous fuel injection 312 so that diesel fuel can effectively ignite the
gas. Injection of first quantity of fuel can begin when the crankshaft is
positioned between about 2 crank angle degrees before TDC and about 5
crank angle degrees after TDC.

[0039] When an engine is idling, and diesel fuel is employed as a pilot to
ignite the gaseous fuel, diesel fuel is injected as the first quantity of
fuel 310 and the combustion of the diesel fuel injected near TDC does
most of the work to overcome friction and parasitic loads to sustain a
predetermined engine idling speed and the combustion of the portion of
diesel fuel injected near the beginning of gaseous fuel injection 312
helps to promote ignition of the gaseous fuel. In this method any fuel
that auto-ignites within the combustion chamber can be injected as the
first quantity of fuel 310. Such a fuel can be selected from the group
consisting of dimethylether, bio-diesel and kerosene. Even though diesel
fuel is referred to herein as a pilot fuel, the diesel fuel injected as
the first quantity of fuel 310 serves both for igniting the gaseous fuel
and for sustaining a predetermined engine idling speed as explained
above.

[0040] When the engine uses an igniter other than a pilot fuel, to promote
ignition of the gaseous fuel, such as a glow plug or other hot surface,
or a spark plug, the first quantity of fuel 310, can be the gaseous fuel
and the gaseous fuel injected near top dead center is combusted to do
work and overcome friction and any parasitic loads, and thereby sustain a
predetermined engine idling speed. The gaseous fuel injected in the later
portion of the first quantity of fuel 310 burns to help ignite gaseous
fuel injection 312.

[0041] With reference still to FIG. 3, in embodiment B of the presently
disclosed method, the injection of the first quantity of fuel is divided
into two pulses such that a first pulse 314 is injected into the
combustion chamber near top dead center and second fuel pulse 316 is
injected into the combustion chamber near the start of gaseous fuel
injection 318. In this embodiment the injection of first pulse 314 can
start at a crank angle of between 2 degrees before TDC and 5 degrees
after TDC. Gaseous fuel injection 318 starts later in the engine cycle at
a crank angle determined through experimental tests to increase the
temperature of the exhaust gas to the operating temperature range for the
SCR converter, generally above 200 degrees Celsius. Higher temperatures
in the SCR converter are generally associated with higher conversion
efficiencies and like in all embodiments, longer delays in the timing for
beginning gaseous fuel injection 318 result in higher temperatures for
the exhaust gas exiting the combustion chamber and with the disclosed
method temperatures higher than 200 degrees Celsius can be achieved as
long as the delay is not so long as to result in the unburned fuel
concentration in the exhaust gas exceeding a predetermined level,
generally, 1000 ppm, or in preferred embodiments 200 ppm or 300 ppm.

[0042] Combustion of first fuel pulse 314 serves to overcome friction and
satisfy parasitic loads to sustain a predetermined engine idling speed
and combustion of second fuel pulse 316, injected near the beginning of
gaseous fuel injection 318, contributes to the ignition of gaseous fuel
injection 318. The end of second pulse 316 generally occurs within
1-degree crank angle before or after the beginning of fuel injection 318
such that the ignition of fuel injected in pulse 316 can effectively heat
the combustion chamber and contribute to the gaseous fuel ignition.

[0043] In alternate embodiments C and D of the presently disclosed method,
injection of the first quantity of fuel is divided into a plurality of
pulses, by injecting respective first pulses 320 and 330, into the
combustion chamber near TDC, followed by one or more respective pulses
322, 332 and 334, as shown in FIG. 3, injected sequentially into the
combustion chamber before a last respective fuel pulse 324 and 336, which
is injected into the combustion chamber shortly before respective gaseous
fuel injections 326 and 338, so that the end of respective pulses 324 and
336, is near the start of respective gaseous fuel injections 326 and 338.
The end of respective last pulses 324 and 336 generally occurs within
1-degree crank angle before or after the start of respective gaseous fuel
injection 326 and 338, as illustrated in FIG. 3. Pulses 320, 322 and 324
and respectively 330, 332, 334 and 336 can each be of the same duration
or can vary in duration. The preferred duration of each pulse and the
separation time between these pulses can depend upon the size of engine,
the type of fuel used and the engine's desired operating characteristics,
but these parameters can be determined empirically by known calibration
methods. Once an engine has been calibrated, the calibrated parameters
can be entered into look up tables or multi-dimensional maps which are
then stored in the memory of controller 116 to be used for controlling
the fuel injection strategy for all engines made with the same design.

[0044] The quantity of fuel injected in first injection 310 and in second
injection 312 in embodiment A are each controllable by controller 116 in
response to the engine speed or any existent load communicated from the
vehicle controller. Similarly, when a plurality of pulses are injected
into the combustion chamber as described in embodiments B, C or D of the
present method, the quantity of fuel injected into the combustion chamber
in the first pulse (for example, pulse 314, 320 or 330) and the quantity
of gaseous fuel (injected for example in injections 318, 326 and 338) are
controllable by controller 116 in response to the engine speed or any
current engine load communicated directly to controller 116 or indirectly
through a vehicle controller. Such variations of engine speed and load
are generally minimal due to the fact that during idling the engine speed
stays low, around 700 ppm, and the engine load is negligible considering
that the engine only needs to overcome friction and parasitic loads,
which are generally very small relative to the engine's maximum load
output.

[0045] When gaseous-fuelled internal combustion engine systems comprising
a SCR converter, as illustrated in FIG. 1, operate at idle according to
the prior art method illustrated in FIG. 2, the temperature of the
exhaust gas is not high enough to maintain the SCR catalyst bed
temperature above 200 degrees Celsius which results in a reduced
conversion efficiency for the SCR converter. The presently disclosed
method illustrated in FIG. 3 differs from the conventional method of fuel
injection used for diesel and direct-injection gaseous-fuelled internal
combustion engines at idle in that the start of fuel injection occurs
later in the engine cycle. More specifically the injection of the first
quantity of fuel starts near top dead center and the injection of gaseous
fuel starts after top dead center, when the piston is on its expansion
stroke within the cylinder. The first quantity of fuel can be a fuel that
reliably auto-ignites, such as diesel fuel or it can be the gaseous fuel,
if the engine is equipped with an igniter in the combustion chamber to
assist with ignition of the gaseous fuel. For example, the igniter can be
a glow plug or other hot surface device, or a spark plug. By delaying
injection of fuel compared to prior art gaseous-fuelled engines, the late
combustion of the gaseous fuel generates less work on the piston and
results in more heat being transferred to the exhaust gas stream. Some of
the heat in the exhaust gases is transferred to the SCR converter in the
after-treatment system and this helps to keep the temperature of the SCR
catalyst bed above a predetermined temperature that results in more
efficient NOx conversion rates; this predetermined temperature can vary
depending upon the catalyst composition, but using known catalyst
compositions this predetermined temperature has been found to be
generally around 200 degrees Celsius.

[0046] It is important to note that gaseous-fuelled engines and
conventional diesel-fuelled engines, which are not fuelled with any
gaseous fuel, have distinct differences, which prevent methods used by
conventional diesel-fuelled engines from being directly applied to
gaseous-fuelled engines. With a conventional diesel engine, the unburned
diesel fuel exiting the combustion chamber which reaches the
after-treatment subsystem is oxidized on the oxidation catalysts of the
exhaust gas after-treatment subsystem generating heat which further
increases the temperature of the exhaust gas. Accordingly, the presence
of excessive unburned diesel fuel in the exhaust gas exiting the
combustion chamber does not result in any adverse effect on the
aftertreatment subsystem and can even be beneficial by raising the
temperature in the aftertreatment subsystem. This is different from
gaseous-fuelled engines, where unburned fuel which consists mainly of
lighter hydrocarbons such as methane, which does not oxidize easily on
the after-treatment catalysts especially at lower temperatures is being
released unburned into the atmosphere through the tailpipe without
generating heat to raise the temperature in the aftertreatment subsystem.
Therefore, in a gaseous-fuelled internal combustion engine system it is
preferable to avoid expelling unburned fuel from the combustion chamber.
The presently disclosed method teaches delaying the injection of gaseous
fuel to begin later in the expansion stroke, while limiting this delay in
order to combust substantially all of the gaseous fuel within the
combustion chamber. The timing for beginning injection of the gaseous
fuel can be determined empirically by known engine calibration methods,
and can depend on various factors such as the engine size and type of
fuel. Such calibration methods were used with a Westport GX 15 liter
engine fuelled with natural gas to produce experimental data that showed
that a preferred timing for the gaseous-fuel injection occurs after the
injection of a diesel pilot fuel, when the crankshaft is at a crank angle
between 14 and 25 degrees after TDC.

[0047] The presently disclosed method of controlling fuel injection timing
to maintain the SCR catalyst bed temperature above 200 degrees Celsius
can be complemented by a preferred air handling strategy. Referring once
again to the embodiment shown in FIG. 1, when controller 116 controls
valve 128 and turbocharger 138 to reduce the cross-section through which
the exhaust gas flows, the result is a reduction in the turbocharger
efficiency which causes an increase of the pumping work of the engine to
maintain a desired power. To generate more pumping work, the fuelling
amount supplied to the engine has to increase. As a consequence, more
fuel is combusted within the combustion chamber generating more heat
which is partially transferred to the exhaust gas exiting the combustion
chamber and thereby increasing its temperature which is in turn
transferred to the catalyst bed in the after-treatment subsystem.

[0048] A Westport GX 15 liter engine fuelled with natural gas was employed
to validate the disclosed method and to compare the engine's emissions
against government regulations. FIG. 4 shows a plot of the engine speed
shown by line 410 and torque output shown by line 420. The hatched areas
show when the engine was idling. The presently disclosed method was
applied in zones 430, when engine speed 410 and torque output 420 had low
values.

[0049] To demonstrate the effect of the presently disclosed method on the
SCR catalyst bed temperature, line 440 is plotted beneath the engine
speed and torque curves. Line 440 is a plot of the SCR catalyst bed
temperature during the engine test corresponding to the torque and speed
curves, when the presently disclosed injection strategy was implemented
in zones 430. Line 440 shows that a short time after the beginning of the
test (around 25 seconds) the SCR bed temperature rose to a temperature
above 200 degrees Celsius, and after this initial rise in the SCR bed
temperature, the temperature was maintained above 200 degrees Celsius for
the entire test, including during the subsequent times when the engine
was operating at idle (zones 430). Line 450 is a plot of the SCR catalyst
bed temperature during a test performed under the same conditions on the
same engine when the conventional method of fuel injection illustrated in
FIG. 2 was applied, showing that the SCR catalyst bed remained at a
significantly lower temperature for an extended time, which resulted in
significantly higher levels of NOx in the tailpipe emissions.

[0050]FIG. 5 illustrates a speed-fuelling map for a Westport GX 15 liter
gaseous-fuelled direct injection engine where the full load fuelling
curve is identified by reference numeral 510. This map also shows a zone
520 that represents the zone on the map where the engine is considered to
operate at idle and where the presently disclosed method of fuel
injection control could be used. The predetermined range of engine speed
and total fuelling amount of zone 520 where the engine is considered to
operate at "idle" can vary from one engine to another depending on the
engine size and type. Generally most engines are considered to operate at
"idle" when the engine speed is around 700 rpm and the values of total
fuelling are in a range at the lowest end of the scale on the map. When
the engine is idling the load on the engine is mainly caused by friction
and parasitic loads. Examples of parasitic loads include, belt driven
auxiliary equipment such as fuel pumps, pumps for engine cooling systems
and hydraulic systems, alternators for producing electrical energy, air
conditioning, and refrigeration units.

[0051] Another zone on the map is zone 540 where the engine is operating
at load. For a truck engine, zone 540 represents the load when the engine
is working to propel the truck. In zone 540 on the map fuel injection
into the combustion chamber can be controlled according to conventional
methods known in the prior art and illustrated in the examples of
mid-range speed (2) and higher speed (3) in FIG. 2. This is because at
higher loads, when more fuel is being combusted, with the fuelling
methods shown in embodiments (2) and (3) in FIG. 2, the temperature of
the exhaust gas exiting the combustion chamber is above 200 degrees
Celsius without needing to delay combustion of the gaseous fuel to
elevate exhaust gas temperatures.

[0052] For an engine used to power a vehicle, a speed-fuelling map as the
one illustrated in FIG. 5 can be stored in the vehicle controller's
memory. The vehicle controller monitors the engine speed and other
parameters indicative of the engine condition, as for example the total
fuelling amount, and communicates the values of these parameters to
controller 116 which controls the fuel injection into the combustion
chamber. When the detected engine speed and total fuelling amount is
within zone 520 on the map, controller 116 controls the fuel injection
according to the present method. When transitioning from engine idle zone
520 to load zone 540, more specifically when the engine speed and the
total fuelling amount are within the boundaries of zone 530, the engine
operates in a transition mode described below.

[0053] If at idle fuel injection is controlled according to embodiment A
shown in FIG. 3, during the transition mode the method comprises
gradually advancing the timing for beginning injection of first quantity
of fuel 310 and the timing for beginning injection of gaseous fuel 312
until each begins when the crankshaft reaches a crank angle before top
dead center which is determined based on the engine speed according to a
conventional method characteristic to the engine operation at load. The
method further comprises over the same period of time, controlling the
quantity of fuel injected into the combustion chamber during injection of
first quantity of fuel 310 and during injection of gaseous fuel 312 until
the quantity of fuel injected in each injection is commanded based on the
engine load according to a conventional method characteristic to the
engine operation at load. The transitioning from the present method to an
injection control strategy similar to those presented in FIG. 2, examples
(2) and (3) can be done over a predetermined period of time. In other
embodiments the engine can operate in the transition mode for as long as
the commanded total fuelling amount and engine speed are within zone 530
on the speed-fuelling map.

[0054] If at idle fuel injection is controlled according to embodiment B
shown in FIG. 3 and controller 116 determines that the engine has
transitioned from idling to load, more specifically when the engine
starts to operate in zone 530 on the speed-fuelling map, the amount of
fuel injected in second pulse 316 and the separation time between the end
of first pulse 314 and fuel injection 318 are gradually decreased until
the amount of fuel injected in the second pulse 316 is close to zero, and
the separation time between first fuel pulse 314 and fuel injection 318
reaches a value that corresponds to the separation time between the first
fuel injection and the second fuel injection determined according to a
conventional method of controlling the fuel injection at load. Over the
same period of time, controller 116 gradually advances the timing of
first pulse 314, second pulse 316 and of fuel injection 318 to promote a
smooth transition to zone 540. The transitioning from the present method
to an injection control strategy similar to those presented in FIG. 2,
examples (2) and (3) can be done over a predetermined period of time. In
other embodiments the engine can operate in the transition mode for as
long as the commanded total fuelling amount and engine speed are within
zone 530 on the speed-fuelling map.

[0055] Similarly, if at idle fuel injection is controlled according to
embodiment C or D shown in FIG. 3 and controller 116 determines that the
engine has transitioned from idling to load, more specifically when the
engine starts to operate in zone 530 on the speed-fuelling map, the
timing for beginning injection of each fuel pulse injected into the
combustion chamber, for example, pulses 320, 322 and 324 and,
respectively 330, 332, 334 and 336 and the timing for beginning injection
of gaseous fuel 326, and respectively 338, are gradually advanced to an
earlier timing in the engine cycle which corresponds to the fuel
injection timing determined according to a conventional method of
controlling fuel injection at load. Over the same period of time
controller 116 controls the fuel injector to gradually decrease the
number of pulses and the amount of fuel injected in fuel pulses 322 and
324 and respectively in fuel pulses 332, 334 and 336 while decreasing the
separation time between the end of first fuel pulse 320 and the start of
fuel injection 326 and respectively between the end of first fuel pulse
330 and gaseous fuel injection 338 until a single first pulse is injected
into the combustion chamber before and near the beginning of the gaseous
fuel injection and the quantities of fuel injected in the first injection
and in the gaseous fuel injection have the values that correspond to the
quantities of fuel determined according to a conventional method of
controlling fuel injection at load. The transitioning from the present
method to an injection control strategy similar to those presented in
FIG. 2, examples (2) and (3) can be done over a predetermined period of
time. In other embodiments the engine can operate in the transition mode
for as long as the commanded total fuelling amount and engine speed are
within zone 530 on the speed-fuelling map.

[0056] As described above it is in this transition zone 530 illustrated on
the speed-fuelling map that the injection control strategy changes from
the present method to an injection control strategy similar to those
presented in FIG. 2, examples (2) and (3).

[0057] While particular elements, embodiments and applications of the
present invention have been shown and described, it will be understood,
that the invention is not limited thereto since modifications can be made
by those skilled in the art without departing from the scope of the
present disclosure, particularly in light of the foregoing teachings.